The present invention relates to an ion source and a mass spectrometer, and to mass spectrometry employing the ion source, as well as to a monitoring system employing the same or to a monitor employing the ion source.
Conventionally, as a method of detecting a minor component in a gas or a liquid with a high sensitivity, it has been known to detect ions generated by ionization in a measuring sample with a high sensitivity by means of a mass spectrometer.
There are various methods for ionizing a sample. One of various sample ionizing methods is an atmospheric pressure chemical ionization method employing a corona discharge. Japanese Patent Application Laid-Open No. 51-8996 (1976) discloses a method in which a sample is introduced in a corona discharge region, which is generated at the tip end of a needle electrode by applying a high voltage thereto, for ionizing the sample. At this time, in addition to the case where the sample is directly ionized by a corona discharge (primary ionization), the sample is also ionized by ion molecule reaction (secondary ionization), resulting in high efficiency ionization of the sample molecule.
On the other hand, as disclosed in Japanese Patent Application Laid-Open No. 6-310091 (1994), among ionization methods which use corona discharge, there has been proposed a method of ionizing a sample without directly introducing a sample gas into a corona discharge region. Namely, the method proposes to use primary ions generated in a separately provided corona discharge region and to efficiently perform secondary ionization of the sample molecules not passing through the corona discharge region by ion molecular reaction. This method is particularly effective for an objective sample, such as silane gas, which cannot be directly ionized using a corona discharge without extreme contamination of the ion source by the discharge product.
Furthermore, there is a method disclosed in U.S. Pat. No. 4,023,398 to French et al. In the disclosed method, a curtain of gas is blown between the corona discharge region and an aperture for introducing ions into the vacuum region in order to prevent introduction of a carrier gas into the mass spectrometric portion which is under vacuum conditions for transporting the sample. Thereby, the discharge efficiency can be improved when a vacuum discharge system, such as a cryopump, is used.
Furthermore, Japanese Patent Application Laid-Open No. 3-179252 (1991) discloses a gas flow direction. In this method, a liquid sample flows through a hollow needle electrode, and a fine droplet of the liquid sample ionized at the tip end of the needle is efficiently vaporized by a dry gas flowing in opposition thereto. On the other hand, dispersion of the ionized sample is suppressed to improve the maintenance efficiency. However, Japanese Patent Application Laid-Open No. 3-179252 (1991) does not disclose a problem in the stability of the discharge, nor a solution of such a problem.
In the method disclosed in the above-identified Japanese Patent Application Laid-Open No. 51-8996 (1976) and Japanese Patent Application Laid-Open No. 3-179252 (1991), when the concentration of the measuring sample is low (for example, upon measurement of a minor component in the air or upon measuring a minor component in a liquid), the intensity of ions of the component present in a greater amount in comparison with the ions of the measuring sample, or ions originated from a component present in a large amount (for example, an ion or the like generated by ion molecular reaction), becomes extremely high. Accordingly, when the sensitivity of a detector is adapted to the ions of the measuring sample in a fine amount, ions of the component present in a large amount or ions originated from the component present in a large amount may reach the detector to cause a large current to flow, resulting in damage to the detector, thereby to gradually degrade the amplification factor of the current.
On the other hand, when a molecule corresponding to a component present in a large amount or a molecule corresponding to ions originated from the component present in a large amount can be ionized easier than the molecule of the objective sample, the generation efficiency of the ions of the objective sample is lowered, thereby lowering the sensitivity. Furthermore, in case of an ion trap type mass spectrometer performing mass spectrometric analysis of accumulated ions by scanning a high frequency voltage after accumulating the ions, ions less than or equal to a mass number corresponding to the amplitude of the high frequency voltage to be applied may directly reach the detector after passing through an ion trap mass spectrometric portion. Accordingly, when ions are present in a large amount, the detector may be damaged, thereby to gradually degrade the amplification of the current.
On the other hand, Japanese Patent Application Laid-Open No. 3-179252 (1991) is related to an ion source for analyzing a liquid sample, but has no disclosure for analysis of gas.
Further, the method disclosed in U.S. Pat. No. 4,023,398, while providing for introduction of a carrier gas into a vacuum by using a curtain of gas, the obtained mass spectrum is nothing different from the prior art. Therefore, problems similar to those of the prior art can be expected.
Furthermore, in the prior art set forth above, the components in the gas to be measured may be deposited on the tip end of the needle, so as to make the corona discharge unstable, thereby to cause difficulty in effecting continuous measurement over a long period of time. In case of the method disclosed in Japanese Patent Application Laid-Open No. 6-210091 (1994), nothing has been discussed with respect to continuous operation over a long period, such as one month.